This invention relates to an apparatus and method for monitoring the amount of thermal power added to or withdrawn from a moving fluid stream, and more specifically, to a non-intrusive apparatus and method which measures the amount of thermal power input or extraction from a flowing fluid by determining the sensible heat capacity rate of the fluid and a temperature differential resulting from the thermal power input or extraction.
There has long been an interest in determining rapidly, economically and easily the thermal power contribution to a flowing stream of fluid from an unknown source, or the thermal power extraction from the fluid stream by an unknown thermal power sink. A myriad of industrial and residential systems employ flowing fluid to which thermal power is either added or taken away. It is quite useful to know the amount of that thermal power added to or extracted from the fluid for determining system efficiencies, losses and capabilities. Thermal energy, which is thermal power multiplied by time, may be considered of more fundamental interest; thermal power and thermal energy are related through the time parameter and can readily be derived from each other.
There are various types of devices used for determining thermal power input into a fluid stream. These devices have tended to be relatively expensive and for that reason are of limited utility.
These prior art devices include intrusive systems. One such system is a null system. In the null system, temperatures upstream and downstream from the unknown source of energy are taken. A heater supplies energy into the stream and the temperatures upstream and downstream of the heater are taken. The energy input of the heater is adjustable to vary the temperature difference upstream and downstream of the heater to equal the temperature difference across the unknown source. When these two temperature differences are the same, the amount of thermal energy introduced into the stream by the heater equals the energy introduced into the system from the unknown source. A null system is comparatively expensive. Null systems also can use too much energy; for example, when the temperature difference across the unknown source is high, a large amount of energy is required to produce the same temperature differential across the heater. Also, when the flow rate is high, a large amount of energy may also be required from the heater to achieve the same temperature differential across the heater. Null systems may also require considerable peak power capacity to enable them to monitor transients, such as a blowdown of a hot storage tank. The large power requirements of a null system can require an intrusive heater, one that is physically in the stream being monitored and not outside the stream's conduit. Null systems can result in or necessitate undesirable alterations in the stream being monitored and complicating procedures. For example, when a large amount of heat energy must be introduced into the stream, it may be necessary to cool the stream to prevent boiling. With high energy input, if heat input is not carefully controlled, the fluid could change state and boil. Further, cooling and then heating the stream results in considerable heat utilization. Null systems also require continual adjustments resulting in complex electronics.
One specific form of null system is disclosed in U.S. Pat. No. 2,398,606 to C. C. Wang, which is specifically designed for ultra-high frequency power measurement. In this particular system, constant temperature ratios are imperative for accuracy.
Intrusive systems, such as the null system, require breaking into the line carrying the fluids. Breaking into the line is clearly a disadvantage.
Heat meters are also in the prior art. One representative of heat meter is described in U.S. Pat. No. 3,167,957 to Ziviani in which heat transferred from a fluid is measured. In Ziviani, an intrusive by-pass duct removes a constant fraction of the fluid from a main stream. This constant fraction is heated in the by-pass duct to a temperature that is a specified function of the amount of heat removed from the fluid in the overall system. The Ziviani patent relies on a flow rate restriction in the main stream which could result in scale build-up in a relatively short period of time.
There are substantial problems with heat meters of this type. A major problem is the necessity to divert fluid and to heat the diverted fluid in proportion to the amount of heat withdrawn by the overall system. It is also exceedingly difficult to obtain a constant fraction of a fluid. In addition, devices of this type use thermocouples which are inherently non-linear. Systems of this type are intrusive because temperature sensors are in the fluid and the by-pass duct must be connected to the main stream for diversion of that stream.
Another heat measuring device is taught in U.S. Pat. No. 2,931,222 to Noldge et al. A fraction of a fluid previously cooled in a heat exchanger is heated to its original temperature. With knowledge of the fraction, the specific heat of the fluid, and the temperature increase, the amount of heat energy taken from the fluid is readily determined. This device is similar to a null system except that it operates in reverse and only on a fraction of the fluid. The system has many disadvantages including the fact that the inlet temperature of the fluid must be known. Further, systems of this type to be accurate must avoid corrosion or scale build-up, and fluid properties, such as viscosity and density, and flow rate, must remain the same; thus, if the viscosity or the flow rate of the fluid changes, the system becomes inaccurate.
U.S. Pat. No. 3,802,264 to Poppendiek et al discloses a meter to determine the flow rate of a stream and utilizes a heater to heat the fluid stream and measures the stream temperature increase. Temperatures are not measured at or beyond the upstream and downstream ends of the power input, but rather intermediate the ends of the power input. In other words, the system of this patent monitors temperature before the heat is uniformly distributed throughout the fluid. With the temperature increase and the power input, the flow rate allegedly can be determined.
Thermally operated flow meters of the type taught in Poppendiek et al are necessarily restricted to laminar flow operation in order to maintain required linearity. Further, the fluid must be homogeneous and it is necessary to know or to be able to derive the specific heat of the fluid. Also, other properties of the fluid, such as density, viscosity and thermal conduction, significantly affect the accuracy, necessitating calibration for each fluid type and input temperature. Consequently, such devices are rather limited in use and in application.
Hot wire anemometry is still another approach for measuring fluid flow. Here, a wire in the stream has power supplied to it. Heat from the wire transfers to the stream. The rate at which heat is lost from the wire is a non-linear measure of stream velocity. The disadvantages of hot wire anemometry include its intrusive character and non-linear output. Hot wire anemometry also depends upon a measure of total heat transfer from the wire and not a measure of a predetermined amount of heat transfer. In other words, hot wire anemometry determines how much heat transfer occurs and does not measure the result of a heat transfer, i.e., temperature change.
A publication entitled "Analog Devices, Multiplier Application Guide," by James Williams et al of Analog Devices, Inc. (1978), teaches a thermally operated flow meter for measuring fluid flow rate. A length of pipe is inserted into or connected to a line carrying the fluid. The pipe includes a resistive heater and upstream and downstream intrusive temperature measuring probes. This device is effective only for measurement of very slow flow rates and requires complex circuitry to enable a constant amount of heat to be supplied to the fluid stream. The device further includes screens located in the pipe or elsewhere in the flow stream to mix the fluid and enhance temperature measurement accuracy.
Flow meters which operate on thermal principles, such as the device in the Poppendiek et al patent, are adapted only for measuring the flow rate of a fluid and are not capable of measuring an amount of heat added to or extracted from a fluid by an unknown heat source or heat sink.